This is very cool – I hadn’t seen Fibonacci done this way before. I was interested in std::exchange too, introduced in C++14. As used here, std::exchange replaces the existing value of j with a new value and returns the original value, which is assigned to i. So this idiom allows you to update a value and store the previous value, all in a single expression.

Even better, Jason shows that this whole program is calculated at compile time in Clang – very impressive.